Lawn mowers include an autonomous lawn mower capable of autonomously traveling without being steered by a worker. This autonomous lawn mower can automatically travel on the lawn by traveling wheels, and mow the lawn by a cutter blade which rotates almost horizontally, and is known as a so-called robotic lawn mower. An autonomous lawn mower like this is known from European Patent Application Publication No. 2687077.
This autonomous lawn mower known by European Patent Application Publication No. 2687077 includes a traveling frame provided with traveling wheels, and a movable cover covering the upper portion and side portions of the traveling frame. The movable cover is held by a plurality of holding devices so as to be displaceable in the horizontal direction and vertical direction with respect to the traveling frame. A displacement amount detector detects the displacement amount with respect to the traveling frame. This displacement amount detector is isolated from and independent of the plurality of holding devices.
Each of the plurality of holding devices includes an accommodating chamber, a vertical shaft, a buffer, and a compression coil spring. The accommodating chamber is formed in the traveling frame. The vertical shaft has an axis vertically extending with respect to the traveling frame, and is accommodated in the accommodating chamber so as to be vertically slidable. The upper end of the vertical shaft is exposed from the accommodating chamber. The buffer is a rubber product having a lower end fixed to the upper end of the vertical shaft, and an upper end fixed to the lower end face of the movable cover. A middle portion of this buffer in the height direction is elastically deformable in the horizontal direction. The compression coil spring biases the vertical shaft downward, thereby pulling the movable cover toward the upper surface of the traveling frame through the buffer. If a side surface of the movable cover hits an obstacle while the autonomous lawn mower is traveling, the buffer elastically deforms in the horizontal direction. As a consequence, the movable cover is displaced in the horizontal direction. The displacement amount detector detects the displacement amount of the movable cover.
To enable the displacement amount detector to accurately detect the displacement amount of the movable cover, the buffer must have a large allowable deformation amount, and a constant deformation resistance with respect to a load in the horizontal direction. However, the buffer is a simple rubber product, so an allowable deformation amount which allows elastic deformation in the horizontal direction is small. To increase the allowable deformation amount of the buffer, it is necessary to increase the length of a middle portion of the buffer in the vertical direction. However, this is not a good plan because the height of the autonomous lawn mower increases. In addition, the rubber buffer changes the deformation resistance with respect to a load in the horizontal direction.
Furthermore, when improving the autonomous lawn mower so that the horizontal displacement amount of the movable cover can be increased with respect to the traveling frame, the following feature must be taken into consideration. That is, when the autonomous lawn mower is traveling, the traveling vibration is transmitted from the lawn surface to the traveling frame through the traveling wheels. If the movable cover and the portion holding the movable cover hit each other due to this traveling vibration, noise may be generated.